1. Field of the Invention
This invention relates to a magnetic disk cartridge. This invention particularly relates to a magnetic disk cartridge in which a magnetic disk is secured to a center core and rotates when the center core is driven by a drive spindle of a disk drive means, said magnetic disk cartridge being kept very small in size.
2. Description of the Prior Art
Magnetic disks having various sizes are accommodated in magnetic disk cartridges having corresponding sizes so that the magnetic disks can be rotated therein. For example, it is desirable that a magnetic disk cartridge which is to be incorporated in a camera, or the like, be kept very small in size. In the case of conventional magnetic disk cartridges accommodating micro floppy disks (having a size of, for example, 3.5 inches), which cartridges are larger than the magnetic disk cartridges incorporated in cameras, or the like, the micro floppy disk is secured to a center core, which is constituted of a metal capable of adhering to a magnet through magnetic attraction. When a micro floppy disk accommodated in a magnetic disk cartridge is rotated in a disk drive means, a magnet secured to a drive spindle of the disk drive means, which rotates the micro floppy disk, magnetically chucks the center core. The center core is provided with two holes, which engage with protrusions on the drive spindle an engagement hole and a second hole called the drive hole. However, for magnetic disk cartridges which are smaller than conventional magnetic disk cartridges accommodating micro floppy disks, it becomes difficult to provide the center core with an engagement hole which can engage with protrusions on the drive spindle of a disk drive means. This is because the center core must be kept very small in size, and therefore it does not have a sufficient area in which to form an engagement hole. Also, with current processing techniques, an engagement hole cannot be formed accurately in such a very small center core.
Accordingly, nowadays there is a need for a very small magnetic disk cartridge having a center core which can engage a drive spindle of a disk drive means easily and reliably.
In order for this need to be met, it has been suggested that the part of the center core of a magnetic disk, which is to be engaged with the drive spindle of a disk drive means, be constituted of a plastic material and molded together with a metal part which is chucked by a magnet of the drive spindle. For this purpose, an insert injection molding, or the like, could be employed. Also, the center core could be provided with a rib which engages with a groove in the drive spindle. In order for the magnetic disk to be rotated reliably when a large force is applied to it, it is desirable that the height of the rib be as large as possible. However, in order for the center core to engage reliably with the spindle and for the magnetic disk to be protected from any damage when the magnetic disk cartridge is loaded into a disk drive means, the height of the rib should be as small as possible. Specifically, if the height of the rib is large, the rib may collide strongly with the drive chucking surface of the drive spindle and break when the magnetic disk cartridge is loaded into a disk drive means. Also, if the rib does not engage with the groove in the drive spindle of the disk drive means and the groove is formed, for example, at part of an annular rib of the drive spindle, the center core to which the magnetic disk is secured will be forced away from its correct position in the magnetic disk cartridge when the magnetic disk cartridge is loaded into a disk drive means. As a result, the magnetic disk will come into contact with the surfaces of the inner walls of the housing of the magnetic disk cartridge and will be scratched or damaged thereby.
Additionally, for very small magnetic disk cartridges, it is necessary for the center core to be rotated very accurately. The accuracy with which the center core is rotated depends on the accuracy with which the center core and the drive spindle engage with each other, and the direction along which the drive force is transmitted from the drive spindle to the center core.